The present invention relates to an optical receiving circuit for converting an input current signal into a voltage signal in an optical communication system.
In recent years, in order to realize future FTTF (Fiber to The Home), researches in an optical subscriber network system are conducted enthusiastically. In the optical subscriber network system, transmission distance differs according to a difference in distances from each home to each station. For this reason, since attenuation of lights in optical fibers also differs, a current signal output from a photodiode becomes a current signal having various amplitudes including an infinitesimal amplitude signal through a large amplitude signal. Therefore, the optical receiving circuit requires a wide dynamic range characteristic which makes it possible to receive the infinitesimal amplitude signal through the large amplitude signal. In order to receive a signal having more infinitesimal amplitude, it is necessary to set a current-voltage converting gain for converting a current signal into a voltage signal to higher value in the optical receiving circuit. However, when the current-voltage converting gain is set to be higher, if a signal having large amplitude is input, an output voltage is saturated, thereby making normal reception difficult.
From such a background, Japanese Patent Application Laid-Open No. 6-85556 (1994) suggests an optical receiving circuit for changing a current splitting amount according to amplitude of an input signal so as to suppress saturation of an output voltage. Namely, in this optical receiving circuit, as the output voltage becomes larger, the current splitting amount is controlled so as to be larger, thereby suppressing the saturation of the output voltage.
As mentioned above, in the system for controlling the current splitting amount according to the output voltage, since the current splitting amount becomes excessive or insufficient because of characteristic fluctuation due to process scattering of a threshold voltage or temperature change in a transistor for current splitting, pulse width distortion, namely, duty deterioration occurs in the output signal. As a result, a clock extracting circuit connected with a later stage malfunctions, and it is difficult to decode received data normally.